Widely known in the art is a gas-discharge high-pressure mercury vapour lamp (JP, A, No. 59-12554), comprising an envelope which accommodates a longitudinally extending inner tube. The envelope is round-symmetrical. One-half of the inner surface of the envelope has a mirror reflector coating applied in such a manner that a plane drawn through extremities of the reflector coating runs in parallel with the longitudinal axis of the inner tube. One part of light radiation of the inner tube passes through a transparent area of the envelope without being reflected from the reflector coating. The other part of the light radiation of the inner tube is incident upon the mirror reflector coating and is reflected therefrom. A part of the light radiation reflected from the mirror reflector coating goes back to the inner tube and is absorbed therein thus lowering luminous eficacy of the gas-discharge lamp.
Also widely known in the art is a high-pressure sodium gas-discharge lamp (DD, A, No. 226429), comprising a cylindrical envelope which accommodates a longitudinally extending high-pressure inner tube. The inner tube is secured to current leads which are sealed in the envolope stem. About one-half of the inner surface of the envelope has a mirror reflector coating applied in such a manner that a plane drawn through extremities of the reflector coating runs in parallel with the longitudinal axis of the inner tube. The inner tube is positioned in the envelope in such a manner that its axis runs in parallel with, hand is offset from, the longitudinal axis of the envelope in the direction towards the surface of the envelope having the reflector coating at a distance which is shorter than one-half of the radius of the cross-section of the envelope. One part of light radiation of the inner tube passes through the transparent area of the envelope without being reflected from the mirror reflector coating. The other part of the light radiation is reflected from the mirror reflector coating and partly goes back to the inner tube to be absorbed therein. The offset position of the inner tube with respect to the axis of the envelope makes it possible to lower the part of light radiation absorbed in the inner tube, but absorption of light radiation reflected from the mirror reflector coating cannot be completely avoided. As a result, luminous eficacy of the gas-discharge lamp is rather low.
Depending on position of the inner tube with respect to the cylindrical surface having a mirror reflector coating, gas-discharge lamps may have different non-round-symmetrical pattern of distribution of light radiation, but the range of possible patterns of distribution of light radiation is extremely limited.